FFF I don't like the fact he change things without say anything.. Pitch lose his memory becouse the nightmares, why the uber cool man on moon didn't say anything to help him? Pitch was a guardian too! :/ Also he could talk to Jack the first time too.. He was just "k your name is jack frost end of the story." Now I'm afraid to say spoilers if I keep talking so I'll shut up XD I just.. idk. Also sorry for bad english.
In the book series, the Guardians of Childhood, it was revealed that he was one of the finest soldiers in the Nightmare Wars that lead into the Golden Age and was appointed the position of security guard of the prison all the nightmares were kept in. However, the nightmares managed to trick him into releasing them by play into his fears of losing his family, specifically his young daughter. In the chaos, Pitch's soul was corrupted to his core and he became the King of Nightmares he is today.